My nephew says no. He says that the air-conditioner is part of the electrical system and doesn’t effect gasoline consumption at all. If you want to know what I mean by significant, guess I mean will you have to fill the tank more in the summer when the air is on constantly, as opposed to the winter.
Cecil approaches the subject:
What gets the better gas mileage: windows up, AC on, or windows down, AC off?
I remembered reading a Cecil column about this when I was in college and that the air conditioning made a very small difference in gas milage but did make it a bit worse. Here is a link to that very article from the archives. I can’t believe that I pulled that almost 20 year old fact from my brain.
Haj
The air conditioner does not use gasoline directly. A belt driven by the engine is required to run the compressor, though. Any drag on the engine IS going to effect gas mileage. My car gets 28mpg w/o the air on and 26 with the air.
Cecil’s storehouse of human knowledge contains the answer to your question 
Yes, running the A/C causes an increase in fuel consumption, but not a huge amount. See Cecil’s column What gets the better gas mileage: windows up, AC on, or windows down, AC off?
Well Depends on the car.
In my Car Manual it states “when running the AC you will experience a drop in gas milage”
Never one to buy everything I read (or read everything I buy)
I decided to log my weekend relaxing drives.
I chose an 80 mile route.
two trips with the AC going full blast
two with the only the drivers side window down.
With AC
trip 1: 67 mpg
trip 2: 66.8 mpg
Without AC
trip 1: 69.2
trip 2: 71.4
Looking at the numbers your thinking WTF is he driving?
It is a 2000 Honda Insight.
As a hybrid, it relies more on electricity for propulsion than the standard vehicle.
Four links to one colum. What is that, some kind of damn record? I knew I should have previewed.
Thanks for your replies. I guess I’m really more interested in idling with the AC on as opposed to waiting in a hot car. Is the answer essentially the same?
Yes, it takes a little bit more power to run the A.C.
I had a car once that would get 29 mpg hwy (sometimes 30) w/o the ac and windows closed and 15 w/ the a/c on. It actually cut it in half and it was very powerful a/c. You could feel a great decrease in power to the car and very cold air comming out of the vents. Sometimes the vents would get frost and if the vents were aimed at the window it would get frost.
I loved that car.
On the urban cycle, I get 50mpg with the AC on, 55mpg with it off.
On the motorway, it’s 58mpg on and 61mpg off.
(according to the readouts on the dash, which I have tested and found accurate)
Unlike Osip this is an ordinary car (well, a diesel engine); it’s a Citroen Xsara Picasso MPV.
It really depends on the car. My Mazda 323 took a heavy hit when the A/C was on. I really had to push down on the gas peddle just to keep the engine running in neutral. (The A/C no longer works. I never ran it enough and the seals dried out. No biggie since it doesn’t get hot much here in Atlanta. Seriously.)
I remember reading Cecil’s column years ago. I still rank it as the most unscientific and specious Straight Dope column ever.
If you don’t run the A/C and get used to the warmth, we’ll all save a huge amount of oil.
Depends on the car. On an older car, the idle is set by the carburetor. If you turn on the AC then the extra load caused by the compressor will simply cause the car to idle at a slightly lower speed. It won’t change the consumption of fuel at all.
On a newer car, the idle is controlled by a computer. As you increase the load on the engine by turning on the AC, the computer compensates and puts the idle speed back up where it thinks it should be.
Either way, your nephew is wrong. The main load on the engine comes from the compressor which is a mechanical device, not an electrical device, although on all cars I’ve taken a close look at the clutch mechanism which disconnects it from the engine when it’s not in use is electrical. And there’s also the matter of the air blower (the little fan usually mounted under the dash somewhere that blows the air into the passenger compartment). It’s electrical, and draws a load whether or not the AC is turned on.
Electrical devices also draw power from the engine. This translates into extra fuel consumption by making the alternator harder to turn, so the engine has to work harder to spin it. Electrical devices may tap the battery as a temporary resevoir (that’s how your engine starts after all) but eventually all of the energy comes from burning fuel.
Heat is the only thing you basically get out of a car for free (except for the electricity used to spin the blower fan). The engine generates heat which as to be taken out of the engine or else it will overheat. If you have the heater off then it is all taken out through the radiator at the front of the car. If you turn the heater on, then some of it goes through a second small radiator in the heating unit. Either way the water pump pushes the same amount of water. It just changes the circulation path a little.
Why do four people link to a Cecil column that is completely inconclusive? You’re better off just typing “i don’t know” and hitting Reply.
Another thing you might ask him is “Where does the electricity come from to power all of the electrical devices on the car?” And when he says “The battery”, you can say “And how does the battery get electricity?”, and when he says “The alternator charges it.”, you can say “And what drives the alternator?”, and he’ll say “Oh yeah…the engine. D’oh!”
In a car, everything needs a source of energy from somewhere.
Exactly. When you put a heavier load on an alternator by turning on electrical components, the alternator becomes more difficult to rotate. That means the engine has to work harder to spin it.
There are no free lunches.
That’s like saying muscles are part of the circulatory system, because blood vessels bring them nutrients.
It may be helpful to realize that auto air conditioning is a system, not a discrete unit like a window air conditioner. The question “how much to replace the car’s air conditioner” is essentially meaningless–there’s no one thing called the air conditioner.
The A/C compressor is run by a belt off the engine’s crankshaft. The compressor clutch is engaged electrically. The blower (inside fan) motor is electric. Many cars also engage the radiator fan motor or a separate A/C condenser fan motor, in either case electric. I’m pretty sure the compressor uses more horsepower than the A/C system’s electrical load on the alternator, but either way the compressor and alternator are loads on the engine that consume gasoline.
I’m sure your nephew means well, but it’s apparent that he has a lot to learn about cars, and also about which subjects he should be making statements about.
Years ago, we were taught that the extra wind resistance from having the windows down consumed more gas than running the A/C. However, we were never given the source of that information. Most likely, it’s true for some cars, but not all. It does take more energy to run the A/C than to run the heater, but there may be other different winter-related factors that affect fuel mileage.
Have you tried clicking on any of our lovely links posted above in the thread, sir? Especially those which link to the writing of a man named, as we like to call him, “Cecil”? We think they may provide some further information on this topic…
Yes, Anthracite, I read Cecil’s column. What I saw was Cecil’s experience using one vehicle, and a contrary experience using one different vehicle. Neither of these could have been the source of what we were told in 1973–Cecil hadn’t written the column, and VW hadn’t gotten beyond the Beetle. It does make me wonder if anyone has studied this with more than one test car.
I had a Rabbit that noticeably lost power when the A/C compressor engaged, whereas in a big American car with a V-8 you usually couldn’t tell. Thus I suspect that one would get different results depending on what proportion of engine power the A/C uses, and on the aerodynamics of windows down vs. windows up for any particular body design.